We brewed an Oatmeal stout (Poppie's Oatmeal Cookie Stout) back in Jan '09 that we've only just now bottled/kegged (there are three of us so we usually bottle to split a 10 gallon batch). We (accidentally) wound up using two different yeasts on each 5 gallon fermenter, so we inadvertently created a chance for a comparison between WLP004 Irish yeast and WLP005 British yeast. The wort was exactly the same, they fermented side by side under identical conditions, aged for the same period of time, and were both force-carbonated at the same pressure, so any difference in taste should be due entirely to the yeasts.

We found the British yeast yielded a slightly drier, slightly more astringent flavor (not to the point of it being a flaw), but a rounder flavor, and showed off more of the raisin flavor (Did I mention we added raisins, molasses, and vanilla to these? We did.) and the roasty flavors of the chocolate malt. Based on White Labs descriptions of both yeasts, I found this to be surprising.

The Irish yeast both smelled and tasted a bit sweeter, and showed off the chocolate-y chocolate malt flavors more. We seemed to be able to taste the Irish batch a little more in the center of the tongue, where the British batch seemed to taste over the entire tongue a bit more. One of our group thought he smelled a slight medicinal/chemical smell to the Irish batch (not enough to really be off), but I think it's just the difference between yeasts.

All in all, a great beer that wound up being two distinct great beers! If we went back to do this recipe again, I think I'd go for the Irish yeast over the British, but my co-brewer would opt for the opposite.

Oh, sorry I don't have any specifics (gravity, recipe etc), but I don't have that in front of me right now. If anyone is really curious, I'd be glad to post both later. Cheers!